Molluscs 2BIbDF09 10/11

What are molluscs?

Molluscs are invertebrates animals that live in water, and mostly in marine environment. There is an estimated number of 100'000 species in the phylum Mollusca. Molluscs appeared in the earliest Cambrian period 545 million years ago. They have a bilateral symmetry and are soft-bodied animals, but most are protected by a hard shell. Some molluscs have external hard shells, like Nautiluses, Snails or Clams, but other have reduced or internal shells, like Cuttlefish or Squids. The rest just lost it during evolution, like Octopuses or Slugs. A shell is not an exoskeleton; it is in fact a protection against predators or the dangers of their environment. If it was an exoskeleton, molluscs couldn't live without it but they can, so they have either a hard shell or an internal skeleton.

We can mainly define four different groups of molluscs; the Gastropods (cf. appendix molluscs 3), the Bivalves (cf. appendix molluscs 4), the Polyplacophores (cf. appendix molluscs 5) and the Cephalopods(cf. appendix molluscs 6). All of them have the same body plan separated in three parts. The first part is the foot which is the strong, muscular portion used for locomotion. The second one is the visceral mass, a soft-bodied portion that contains internal organs and the third one is the mantle, that covers and protect the visceral mass and, for those who have shells, secrets the shell. (cf. appendix molluscs 1)

What to they eat?

Molluscs, as they are separated in groups, don't have the same diet. Some are carnivorous, others herbivorous, filter feeders or detritus feeders. Their diet is bounded with their live style. Cephalopods are carnivorous, gastropods are herbivorous and most bivalves are filter feeders. They mostly have a radula, which is a rasping organ that molluscs use to scrape up food: for example scrape algae off rocks or brake some shells. (cf. appendix molluscs 9) The food is then digest by two organs, like the human digesting cycle(cf. appendix molluscs 1).

Annexe is FRENCH... You should say APPENDIX Pierre.brawand 16 janvier 2011 à 21:09 (CET)

what is a shell? Is it an exoskeleton?

Classification of Molluscs

• Domain: Eukarya

• Kingdom: Animals

• Phylum: Molluscs

• Classes: Bivalves, Cephalopods, Gastropods, Polyplacophores (cf. appendix molluscs 7)

What are the main features of each group?

The Gastropods

The Gastropods form the most important group of molluscs (over 40,000 species). This group include snails, whelks, limpets and slugs. We call them Gastropods because it means stomach-footed. All the gastropods are oviparous animals.

We can find them nearly everywhere in the world because they live either in water or in moist environments. Their anatomy differ a bit with other molluscs, because of their color or their way of respiration, for example.

Most gastropods are herbivores or detritivores. They have a unique structure in their mouth called a radula, which allows them to eat easier.

The radula is a rasping organ which scrape up algae plants cell or other type of food on rocks for example. It is usually covered with teeth. Sea butterflies are gastropods with a reduced or absent shell but with a large winglike foot that flaps and power swimming movements.

How do gastropods move?

Gastropods move by muscle contractions, thanks to their ventrally flattened foot. That is why gastropods that live on land are very slow.

Gastropods can have a shell, like snails.

What are the differences between the Land gastropods and the sea gastropods?

The shell of snails change. Land snails have thinner shell than marine snails. This shell is in a shape of spiral, which is used for protection.

where did you get this information? In a French website? If this is the case, don't do that. I don't want you to translate. I want you to read in English and to synthesize the main information. Pierre.brawand 16 janvier 2011 à 21:11 (CET)

The nudibranchs are marine gastropods which have lost their shell in evolution. We also call them sea slugs.

snails: The snails are hermaphrodite. This means that every snail have a female part and a male part. The female part can produce eggs and the male part produces sperm. It is impossible for only one snail to reproduce itself, the snail has to meet an other one. However during the meeting the snails can change their sexual part when the two snails have their eggs fertilized, so they can leave and lay some eggs after fertilization.

The snail's body is kept moist by mucus which helps this animal to glide along as foot ripples. The mucus which is produced by the snail helps it to move and reduces the risk of wound. When snails pull their bodies into their shells, they can close the operculum, which is like a door to protect themselves. They have two pairs of tentacles of which the longest has eyes in its extremity.

Nudibranchs: Limpets are the most primitive organisms of all the gastropods. they are found in marine environments. Limpets have the same body plan as other molluscs.

The Cephalopods

Cephalopods differ from gastropods and bivalves. Unlike bivalves and most gastropods which have shells, most cephalopods, like the Octopus, miss it. Only a few cephalopods have a heavy external shell (like the Nautilus) or a small and internal shell (like the Squids), which is a cuttlebone; it is neither a bone or a skeleton, but a shell rest. (cf. appendix molluscs 10)

They are carnivorous predators made for speed and agility; cephalopods are know for their intelligence and ability to catch their fast-moving prey. They are found in all oceans of the world. All species of cephalopods have tentacles that they use to catch and hold prey; mostly fishes. The tentacles seem to be arms that help them to feed themselves and cling to surfaces, thanks to suction cups on the tentacles. Cephalopods are great predators thanks to their speed movement, camera eye-vision and their intelligence.

Their complex behaviour makes them comparable to vertebrates rather than other invertebrates.

How do they move?

They move by jet propulsion, which is a complex muscular motion:

The seawater is first drawn into the mantle cavity over the gills. When the animal wants to move, it quickly contracts the mantle wall. The pressure of water within the mantle cavity is increased because there is only one orifice not sealed, the funnel or siphon, by which the water goes out, propeling the animal fast through the surrounding water. (cf. appendix molluscs 8)

How do they see?

The cephalopods need to see quite well so that they can catch prey easily, thus they have what we call a "camera eye vision", which is a great sight they have thanks to their developed eyes, that don't differ so much with human eyes. Indeed, the eye of a cephalopod is composed of a two chamber system, separated by a lens, with a retina that catch light and a pupil with an iris. The structure of the retina show similarities too, but the human eye is more complex. In both, the pupil protects the retina from the amount of light. The octopus's eyes is probably the highly developed light sense organ in the animal's kingdom.

Cephalopods are considered like the most intelligent invertebrates. They have a develop nervous system and an extremely complex brain, which is an advantage being predators. Thanks to their intelligence, they are able to adapt to their environment and Squids have the ability to "disappear", using camouflage as the main protection against predators. They can control their pigment cells (chromatophores) and thus are capable of changing their colour, to protect themselves against predators or hide ambushing prey. (cf. appendix 11)

The Bivalves

The class of Bivalves is a big group too, as we know 10'000 species of them. Most bivalves are sedentary, living in sand or mud in marine and freshwater environments. Bivalves, "bi" meaning two, have shells divided into two halves hinged together.

How do they protect themselves?

They use their sensitive nerves to feel intruders and protect themselves by closing their shells together. Their hard shells help them to protect too. They don’t have any eyes or ears, so they only can feel their environment.

They live in big colonies because they don’t move either fast or for long distances, so they need to stay close so that they can reproduce.

How do they breathe?

Bivalves are have huge gills that enables them to filter food by the water they breathe. This water goes through the mussel cavity and passes on the gills, which are stuck against the walls of the body cavity. It is an extremely important capacity for the water's ecology. Indeed, one blue mussel filters up to 20 litres of water each day, "washing" it.

make a link with the mussel observation made in class Pierre.brawand 30 janvier 2011 à 15:13 (CET)

The Polyplacophores

Polyplacophores are molluscs with many shell plates. We know approximatively 750 species of them. This class is the closest related to the molluscs' common ancestor. Chitons are the best known Polyplacophores. They can live up to 3000 meters depth, in marine environments. Unlike most molluscs, chitons do not have a calcareous shell. As we said, chitons are closely related to the molluscs' common ancestor but they have differences with it. For example, the chiton's cuticula of the mantle, which protects a small part of their back, is only located on its back sides, unlike the molluscs' ancestor. Their whole back is in fact protected by eight calcareous shell plates.

A chiton is flattened and elongated, and can attach itself tightly to rocks or something hard thanks to its strong foot. It usually attaches by a combination of muscular contraction.

(Contrary to snails, for example, chitons, have no mantle cavity containing gills. By far the most chitons are herbivores and live on algae they rasp from the rocks and also from mussels in their vicinity. When doing so, they cling hard to the ground with their powerful foot, supported by the sturdy girdle. Chitons, however, do have a strong longitudinal muscle, that enables them to roll in like a woodlouse A special adaptation to the area it lives in are a chiton's light sense organs. Those can be found in the dorsal shell plates! They enable the chiton to recognize a threat coming from behind without having to expose its vulnerable belly. The majority of chitons have separate sexes. The pair of sexual organs (gonads) usually are grown together forming one organ with two channels leading into the mantle groove. Fertilization takes place outside in the surrounding water. Chiton development passes a larval state resembling a trochophora larva, which by a metamorphosis changes into a young chiton much more like the adult. Some few chiton species (Placiphorella) have changed to a carnivorous way of living. They lift the frontal part of the girdle from the ground an watch for prey, usually small crustaceans. Small tentacles around the mouth detect the prey, which is then caught by firmly pressing the girdle to the ground)

(The latter remain raised until prey come to pass underneath. The chiton will fall back on its prey, trapped, will be slowly consumed.This technique allows them to catch prey, however, much fasterthan them. to change)

What are the main features of animals?

Animals are eukaryotic and multicellular organisms which live on earth, in all environments; water, underground, aboveground, in the air, in plants, etc. They are heterotrophic: they need to ingest food to generate energy and survive. They can move and differ from plants, algae and fungi in lacking rigid cell walls and having a different embryos development.

Are molluscs animals?

Molluscs are eukaryotic and multicellular organisms. They mostly live in water or moist environments on earth. They need to ingest food to survive; either other animals or plants and algae. They can move, even if some molluscs are sedentary and have this particular embryos development which is a characteristic exclusive to animals. Thus, we can confirm that molluscs are animals.

References

• http://www.earthlife.net/inverts/mollusca.html
• Essential Biology with Physiology Campbell/Reece/Simon Second Edition
• http://www.dtmag.com/Stories/Marine%20Life/02-06-whats_that.htm
• http://www.waterencyclopedia.com/Ce-Cr/Cephalopods.html
• http://sharon-taxonomy2009-p3.wikispaces.com/Mollusca
• http://www.weichtiere.at/english/mollusca/index.html

good start but still a lot of work to do...! Pierre.brawand 16 janvier 2011 à 21:13 (CET) co

still a lot to do... Pierre.brawand 30 janvier 2011 à 15:13 (CET)

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